Spatial light modulators presently constitute one of the essential components of most systems for processing information optically.
An essential category of such components is formed by spatial light modulators which are capable of being written to optically and which serve to convert an input light beam or field into an output light beam or field, the output light beam containing a copy of the input information or "image" contained in the input light beam. However, the optical properties of the output light beam are adapted to satisfy the requirements of the optical processing system placed downstream from the modulator. For a more detailed description of such components and the essential characteristics thereof, reference may be made to the work edited by Neil Collings entitled "Optical pattern recognition", Chapter 4 "Spatial light modulators" published by Addison-Wesley Publishing Company ISBN 0 201 14549 9.
In conoscopic holography devices, a birefringent crystal placed between two circular polarizers is illuminated with non-coherent light in order to obtain a holographic image of the image of an initial object. In general, it is considered that the ordinary light beam and the extraordinary light beam derived from the initial image by the birefringent crystal are equivalent to the object light beam and to the reference beam in coherent light holography. For a more detailed description of this type of device, reference may be made, for example, to U.S. Pat. No. 4,976,504. Compared with coherent light holography devices, the above-mentioned conoscopic holography devices have the advantage of simplified implementation of a birefringence crystal by means of circular polarizers, the illuminating beam being a non-coherent monochromatic beam.
However, such devices generate a conoscopic image beam which includes a large unmodulated or "continuous wave" (CW) component inherent to spatial amplitude modulation of the input light beam to make the image containing the input information. Such a CW component suffers from the drawback of greatly limiting the signal to noise ratio of the corresponding conoscopic holography devices. Such a CW component due to the intensity of the input light beam prior to forming the image of the object to be processed by conoscopic holography cannot be eliminated physically either before or after processing by conoscopic holography, since any optical preprocessing followed by conversion into corresponding electronic signals will necessarily degrade the looked-for information obtained by holographic processing.
An object of the invention is to remedy the above drawbacks by implementing a spatial light modulator device that makes it possible to adjust the operation point and to reduce or eliminate the CW component of an input beam while retaining the amplitude dynamic range qualities of the transmitted input beam and of a corresponding read light beam so as to increase the signal to noise ratio on detection.
Another object of the present invention is to implement a spatial light modulator device in which the write light beam may be constituted by a non-coherent light beam while, on the contrary, the read beam may be constituted by a beam of coherent light so as to convert non-coherent information into coherent information for the purpose of reconstructing the coherent information in real time.